It has generally become accepted that expensive apparatus particularly that used in medical and dental applications need be recycled by cleaning and sterilizing before reuse. Much of the apparatus for such purposes requires very special handling in order to insure that it is in fact sterile. It is often not practicable to use thermal means to accomplish such sterilizing and therefore various chemical sterilizing agents have found use. It is of extreme importance that a high degree of confidence exist in the products that are going to be utilized for the sterilizing to insure that they have been properly mixed prior to being actually used for the sterilization purpose. One normally starts with a concentrate of the active materials to be used and dilutes them down with water to the desired level for actual sterilization. It is important that the active material be neither over nor under diluted.
Sterilants of the peracetic acid-hydrogen peroxide types have been known for many years. The sterilants are prepared by mixing acetic acid and hydrogen peroxide to give an equilibrated solution of peracetic acid, acetic acid and hydrogen peroxide. When properly mixed and diluted, these solutions have shown great efficacy in killing not only bacteria but various other microbiological materials. A great deal of literature and patent material exist in dealing broadly with peracetic acid-hydrogen peroxide solutions for sterilization. Without limitation, the reader is directed to U.S. Pat. Nos. Bowing 4,051,058 and 4,051,059. The reader's attention is also directed to the text ORGANIC PEROXIDES by Daniel Swern and to M. G. C. Boldry, "The Bactericidal, Fungicidal and Sporicidal Properties of Hydrogen Peroxide and Peracetic Acid," J. App. Bacteriology 54, 417-423 for further background on the field of the use of peroxides and hydrogen peroxide in microbiological applications.
In U.S. Pat. No. 4,517,081 there is described a machine which cleans and sterilizes dialyzers to enable their reuse. The machine includes means for passing the series of cleaning, sterilizing and rinsing fluids through the compartments of the dialyzer in a predetermined sequence. Predetermined dilutions of stock solutions are accomplished in a fluid tank within the machine which communicates with a microprocessor controlled manifold for introducing controlled amounts of stock solutions and water into the tank. Commercial embodiments of the invention have utilized peracetic acid-hydrogen peroxide solutions of different concentrations for both the cleaning and sterilizing functions.
Peracetic-hydrogen peroxide solutions are inherently unstable and, therefore, subject to variation in concentration of active peroxy components with aging. Highly concentrated solutions are more stable. The high concentration of conventional commercial peracetic acid-hydrogen peroxide solutions must be manually diluted before the solution can be conveniently handled as a stock solution in a machine such as described in U.S. Pat. No. 4,517,081. The pre-dilution introduces possibilities for human error in the dilution and, therefore, in the ultimate concentration and uniformity within the diluted concentrate of peracetic acid and hydrogen peroxide used to clean and sterilize the dialyzer. Should the diluted material be too dilute a serious risk of residual living bacteria remains. Too concentrated may result in insufficient rinsing to remove the sterilant and may even damage the material being sterilized.
Conductivity measurements have been widely used to measure concentrations of solutes in solvents and for measuring other parameters indicated by fluid concentration changes. For instance, blood conductivity measurements have been used to monitor cardiac output as described in U.S. Pat. Nos. 3,985,123, 4,572,206, 4,380,237, and Bourdillon et al, Med. & Biol. Eng. & Comput. 17, 323-329 (1979).
While conductivity measurements to monitor concentrations of solutes in aqueous solutions are well known. It is believed that, heretofore, no such measurements have been used to monitor peroxide/peracid solutions concentrations.